I just donated to help pay Jeff Bauman's medical bills. He's the guy who lost his legs in the bombing, and helped ID the bombers. If you want to contribute you can do it here:
http://www.gofundme.com/BucksforBauman
There are also campaigns for some of the other victims if you're interested.
http://www.giveforward.com/organization/BostonMarathon
http://www.gofundme.com/BelieveinBoston